<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        #box{
            width: 200px;
            height: 200px;
            background: red;
        }
    </style>
</head>
<body>
    <div id="box"></div>
    <script>
        /*
            函数创建完成之后，是需要调用的！！！否则呢不执行
            调用有名函数：函数名();
        */
        function fn() {
            alert(1);
        }

        // fn();

        var box = document.getElementById('box');
        box.onclick = function(){
            //放置代码块
            alert(2);
        }

    </script>
</body>
</html>